1. Field of the Invention
The embodiment discussed herein is related to a semiconductor device and a semiconductor device fabrication method.
2. Background of the Related Art
Power semiconductor modules (semiconductor devices) include semiconductor elements, such as insulated gate bipolar transistors (IGBTs), power metal oxide semiconductor field effect transistors (MOSFETs), or free wheeling diodes (FWDs), and are widely used as power converters or the like.
Semiconductor devices include a plurality of semiconductor units and increases current capacity. With such semiconductor units, a back electrode of a semiconductor element is joined to metal foil disposed over an insulating board with solder therebetween. In addition, with the semiconductor units, a conductive post soldered to a through hole made in a printed circuit board is joined to a front electrode of the semiconductor element with solder therebetween. These insulating board, semiconductor element, and printed circuit board are sealed by the use of resin to form each semiconductor unit.
Furthermore, these semiconductor units are fixed onto a base plate with solder there between and are housed in a case. By doing so, a semiconductor device is formed.
Please see, for example, Japanese Laid-open Patent Publication No. 2009-64852.
In order to fix a semiconductor unit onto a base plate with solder there between, first a solder plate is disposed over the base plate. Furthermore, the semiconductor unit is disposed over the solder plate. All of them are then heated. By doing so, the solder plate is melted and the semiconductor unit is fixed onto the base plate by the use of solder.
However, when such heating is performed, solder by which metal foil and a semiconductor element included in the semiconductor unit are joined together or solder by which the semiconductor element and a conductive post included in the semiconductor unit are joined together is remelted. Remelted solder reacts with an electrode of the semiconductor element to form an alloy layer. That is to say, an electrode of the semiconductor element is eroded by remelted solder and is consumed (solder erosion). As a result, a deterioration in the quality of the joining of the semiconductor element may lead to a deterioration in the characteristics of the semiconductor unit and therefore to a deterioration in the quality of a semiconductor device.